Within the enormous group consisting of carbonaceous compounds is a set of pre-graphitic compounds that are generally prepared at low temperatures (eg: less than about 2000.degree. C.) from various organic sources and that tend to graphitize when annealed at higher temperatures. There are however both hard and soft pre-graphitic carbon compounds, the former being difficult to graphitize substantially even at temperatures of order of 3000.degree. C., and the latter, on the other hand, being virtually completely graphitized around 3000.degree. C.
The aforementioned set of compounds has been of great interest lately for use as anode materials in what is called lithium-ion or rocking chair type batteries. These batteries represent the state of the art in small rechargeable power sources for commercial electronics applications. Typically, these batteries have about twice the energy density (Wh/L) of conventional rechargeable systems (such as NiCd or lead acid batteries). Additionally, lithium ion batteries operate around 3 1/2 volts which is often sufficiently high such that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Insertion compounds are those that act as a host solid for the reversible insertion of guest atoms (in this case, lithium atoms). The structure of the insertion compound host is not significantly altered by the insertion. In a lithium ion battery, lithium is extracted from the anode material while lithium is concurrently inserted into the cathode on discharge of the battery. The reverse processes occur on recharge of the battery. Lithium atoms travel or "rock" from one electrode to the other as ions dissolved in a non-aqueous electrolyte with associated electrons travelling in the circuit external to the battery.
The two electrode materials for lithium ion batteries are chosen such that the chemical potential of the inserted lithium within each material differs by about 3 to 4 electron volts thus leading to a 3 to 4 volt battery. It is also important to select insertion compounds that reversibly insert lithium over a wide stoichiometry range thus leading to a high capacity battery.
A 3.6 V lithium ion battery based on a LiCoO.sub.2 / pregraphitic carbon electrochemistry is commercially available (produced by Sony Energy Tec.) wherein the carbonaceous anode can reversibly insert about 0.65 Li per six carbon atoms. (The pre-graphitic carbon employed is a disordered form of carbon which appears to be similar to coke.) However, the reversible capacity of lithium ion battery anodes can be increased by using a variety of alternatives mentioned in the literature. For example, the crystal structure of the carbonaceous material affects its ability to reversibly insert lithium (as described in J.R. Dahn et al., "Lithium Batteries, New Materials and New Perspectives", edited by G. Pistoia, Elsevier North-Holland, p1-47, (1993)). Graphite for instance can reversibly incorporate one lithium per six carbon atoms which corresponds electrochemically to 372 mAh/g. This electrochemical capacity per unit weight of material is denoted as the specific capacity for that material. Graphitized carbons and/or graphite itself can be employed under certain conditions (as for example in the presentation by Matsushita, 6th International Lithium Battery Conference, Muenster, Germany, May 13, 1992, or in U.S. Pat. No. 5,130,211).
Other alternatives for increasing the specific capacity of carbonaceous anode materials have included the addition of other elements to the carbonaceous compound. For example, European Patent Application No. EP486950 and Japanese Application Laid-Open No. 03-245458 mention the addition of small amounts of phosphorous and boron respectively to enhance the anode specific capacity. The mechanism behind this effect is unclear but it may be a result of modifications to the microstructure of the carbonaceous compound. Also, Canadian Application Serial No. 2,098,248 discloses a means for enhancing anode capacity by substituting electron acceptors (such as boron, aluminum, and the like) for carbon atoms in the structure of the carbonaceous compound.
Most recently, two groups have prepared carbonaceous materials with very high reversible capacity by pyrolysis of suitable starting materials. K. Sato et al. in Science 264, 556, (1994) disclosed a carbonaceous material prepared by heating polyparaphenylene at 700.degree. C. which has a reversible capacity of 680 mAh/g. At the Seventh International Meeting on Lithium Batteries, Extended Abstracts Page 212, Boston, Mass. (1994), A. Mabuchi et al. disclosed a low density (about 1.5 g/cc) carbonaceous material prepared by heating coal tar pitch at 700.degree. C. which has a reversible capacity of about 750 mAh/g. These values are much greater than that of pure graphite. However, both materials have a very large irreversible capacity as evidenced by first discharge capacities of over 1000 mAh/g for the former and about 1200 mAh/g for the latter. Both materials also are crystalline enough to exhibit x-ray patterns from which the parameters d.sub.002, L.sub.c, a, and L.sub.a can be determined. Neither material therefore incorporates additional elements (such as electron acceptors) and neither material is amorphous based on x-ray diffraction. It is unknown yet why these carbonaceous materials exhibit such high capacity.
Historically, however, metallic lithium was preferred as an anode material during development of rechargeable lithium batteries. Lithium metal has a specific capacity of 3.86 Ah/g, significantly greater than presently known alternatives. There are however numerous problems associated with the use of metallic lithium as an anode, most notably its poor safety record in larger battery sizes (of order of AA size or greater). The use of lithium metal anodes in rechargeable batteries has effectively been limited to very small consumer configurations (such as coin cells) or to military applications and the like.
Anode materials other than pure lithium have also been proposed and include a class of lithium alloys such as those listed in the following Table 1. Mixed alloys have also been proposed as illustrated in Y. Toyoguchi et al., Progress in Batteries and Solar Cells, 6, 58 (1987). As shown in Table 1, lithium alloys can comprise significant amounts of lithium that can be extracted and re-alloyed in principle. However, in practice, there are large volume charges associated with varying the stoichiometry of lithium in these alloys between the limits shown. These volume changes have several effects in a battery application. Firstly, the alloy anode tends to crack and fragment upon repeated alloying/extraction cycles which reduces the anode to "dust". This can result in integrity problems for the anode. Secondly, as a result of said fragmentation, the surface area of the anode increases. Since these materials are close to the chemical potential of lithium, the lithium within the alloy gets continually consumed via reaction with the battery electrolyte in the formation of passivating films on newly exposed anode surface. This reaction is undesirable since it consumes lithium irreversibly, thus resulting in overall capacity loss in a battery. Finally, an increase in surface area of a reactive anode can lead to increased sensitivity to thermal runaway, a major safety concern.
TABLE 1 ______________________________________ LI-ALLOY ANODE MATERIALS FOR SECONDARY LI BATTERIES Specific Capacity Average Range mAh/g of Voltage Reference* Material of x alloying element versus Li (V) * ______________________________________ Li.sub.x Sn 0.4-4.5 902 0.5 2 Li.sub.x Al 0.0-1.0 992 0.3 3 Li.sub.x Si 0.0-4.2 4017 0.2 4 Li.sub.x Cd 1.0-3.0 476 0.07 2 Li.sub.x Pb 1.0-4.4 440 0.2 4 Li.sub.x Bi 0.0-3.0 384 0.8 4 Li.sub.x Sb 0.0-3.0 658 0.95 4 Li.sub.x C.sub.6 * 0.0-1.0 372 0.1 1 ______________________________________ *Denotes an intercalation compound for comparative purposes. **Reference 1 is J. R. Dahn et al. in "Lithium Batteries, New Materials and New Perspectives", edited by G. Pistoia, Elsevier NorthHolland, p1-47 (1993). Reference 2 is A. Anani et al., Proceedings of the Electrochemica Society, 871, 382-92 (1987). Reference 3 is J. Wang et al., Solid State Ionics, 20, 185 (1986). Reference 4 is R. A. Huggins, Proceedings of the ElectrochemicalSociety 871, 356-64 (1987).
The use of carbonaceous insertion compounds as anodes has avoided the aforementioned problems with lithium alloy anodes. Since the volume changes associated with lithium insertion are small, little or no fragmentation of the carbonaceous compounds occurs. Thus, anode integrity can be maintained more easily and the anode surface area can be kept from increasing. No significant capacity loss due to further passivation film formation on fresh surfaces need occur and the thermal stability of the battery need not worsen with cycle number. Commercial batteries with carbonaceous insertion compound anodes have achieved over a thousand charge-discharge cycles without significant capacity loss and with an actual slight improvement in safety to abuse (as shown in K. Ozawa et al., The Tenth International Seminar On Primary And Secondary Battery Technology And Application, Mar. 1-4, 1993, Deerfield Beach, Fla.).
Co-pending Canadian Patent Application titled `Carbonaceous Host Compounds and Use as Anodes in Rechargeable Batteries` filed May 3, 1994 discloses carbonaceous insertion compounds that comprise a pregraphitic carbonaceous host, having both organized and disorganized structural regions, and atoms of an element capable of forming alloys with an alkali metal. The alloying atoms are incorporated into the carbonaceous host without substantially affecting the structure of the organized regions, as evidenced by x-ray diffraction measurements. The alloying atoms can be incorporated predominantly as monodispersed atoms in the disorganized regions of the host. (The term `monodispersed` is intended to include single atoms and/or small clusters of the alloying element such that the resulting compound exhibits properties more characteristic of single atoms of the alloying element than that of a bulk compound of the alloying element.) Certain compounds of this invention are attractive for use as anode materials in lithium ion batteries. When the alloying atoms are silicon for example, the reversible capacity of the compounds of the invention can be increased over that of the carbonaceous host alone while still maintaining cycling performance similar to the carbonaceous host. Thus, it appears that the compounds of the invention can combine to some extent the attractive features of the capacity of the alloying element with the cycling stability of a carbonaceous compound.
Example compounds in the aforementioned Canadian patent application were prepared by chemical vapour deposition methods although it was expected that similar results could be obtained by pyrolysis of suitable polymer precursors. Other carbonaceous compounds with high Specific capacity have been prepared by such pyrolysis techniques. For example, Sony Energy Tec (in European Patent Publication Number 357,001) has reported preparing a carbonaceous compound containing phosphorus with a specific capacity of about 450 mAh/g by pyrolyzing polyfurfuryl alcohol. The polyfurfuryl alcohol in turn had been prepared from the monomer polymerized in the presence of phosphoric acid.